Most of the time you can book hotels, flights, buses, trains, cruises and car rentals directly through the aggregator (who may receive a commission for your booking) but there are also those that only list prices and direct you to other pages for booking. While aggregators are not strictly essential and goods or services booked through them can be more expensive than those booked directly, they can be a godsend in weeding through countless offers for example in air travel.

For hotels, cruises, etc., only the major players may be part of the comparison. Thus, especially for off the beaten track destinations, you might want to use other means to find local businesses.

Aggregators are notorious for displaying different prices to different customers based on browsing history, past purchases, and other inscrutable factors. Reportedly, it helps if you clear your cookies or switch your browser to "private browsing" mode before opening one of these websites.

Various goods and services when booked online can be gotten with a certain "cashback": a website gets a cut for every sale they enable and they hand on a part of that cut to its users. Cookies from an aggregator search can cause the cashback to be rejected, so surf with caution.

Depending on a variety of factors, booking through an aggregator may be the only way to ensure you'll be rebooked onto the next flight/train/bus in case of a delay that lets you miss the originally planned connection. If all legs of the trip are with the same company or alliance, this protection usually exists when booking directly through them, but if you combine different companies that is often not the case. Some aggregators explicitly guarantee their connections (even if booked on different companies) when booked through them. Sometimes with added caveats like "hand luggage only". If such a guarantee is explicitly given, they are of course bound by it, otherwise the territory gets very murky.

Nowadays air travel is only rarely booked directly through the airline without first searching and comparing prices. Sometimes the same flight can have vastly differing prices at various agregators and it pays to compare search results and to also look at the website of the airline itself before booking.

Rail air alliances, especially rail&fly are frequently difficult or impossible to book through aggregators and the vast majority of them won't know IATA codes for railway stations or rail&fly as a whole (QYG). You might have better luck directly with the airline, but sometimes the easiest way to book stuff like this is to actually go talk to an actual human. Given that rail&fly must be booked with your ticket for most flights (some airlines allow it to be added later on, but not even all airline staff know that) and offers both a better price and more flexibility than booking a train trip separately, this is no small thing, especially when you know the price difference to be considerable. Another frequent annoyance with air travel aggregators is that they often want to upsell or offer you hotels, rental cars and the like that you might not need or want all things considered or that can be gotten cheaper through other sources. Aggregators are also known to use cookies to see which flight routes you want to buy, charging you more for a flight you desperately want to take: clear cookies, use private or incognito browsing or search from a different browser or computer to diminish this issue.

If you are flexible with your dates, some aggregators allow searches for +/- three days and others display prices for different departure dates, but you'd have to run the search again to see how accurate those are. Some airlines, most notoriously Ryanair and Southwest, may not show up in aggregator searches, so it might pay to compare with those as well.

Kayak — a popular aggregator, good at finding cheap flights, with lots of options to modify your search.

Momondo — reliably finds the cheapest (or close to the cheapest) option available. Good user interface. Allows you to search by frequent flier alliance if you're looking to accumulate miles.

Google Flights — fast but doesn't always find the cheapest options. Has features that show you the best dates to fly if your dates are flexible and the cost of different destinations if you haven't yet decided where to go.

Skyscanner — includes some airlines that don't usually appear in aggregators, and allows you to set up to 30 "watchdogs" to search for unusually cheap fares (often errors by airlines, which can allow you to travel very cheaply if you act fast). Also lets you search without specifying a destination so you can see what destinations are cheapest for the chosen dates. "Whole month" feature does not usually work on secondary destinations.

Flightnetwork — pairs several low cost airlines that normally other aggregators do not pair making it a great option for international routes.

Kiwi.com — allows search from multiple airports to multiple airports, on flexible dates. Sells tickets at higher prices, but it is possible to just find the flight and buy it elsewhere

Tripchemy — fast and cheap multi-city round trip search engine for destinations in or around Europe. You buy the tickets yourself using Google Flights. Great if you want to visit multiple destinations on a budget.

To find a low-cost/no-frills flight it can be good to check one of the comparison tools, such as, e.g., flylowcostairlines.org.

Sites such as Expedia and Travelocity can help you explore your options, but these may not show budget airline flights, and they are rather North America-centric, often showing ridiculously inflated (full-fare) prices for travel outside North America.

Busradar ("Busliniensuche" in German) by far the best, financed by ads for BlaBlaCar (a ridesharing service) and Deutsche Bahn. It's up to you whether you want prices for those alternatives to buses to be displayed. Keep in mind that the DB fares listed on the site are always without BahnCard discount, so if you have one, you may actually get a cheaper price at the DB website. For booking, you are always redirected to the website of the bus or train company or BlaBlaCar.

Prices can be shown in Zloty, Czech Crowns and British Pound Sterling instead of Euros.

9292 allows you to check and search for your connections by public transit (trains, buses, trams and metros are covered) and gives you an easy overview of the quickest, easiest, earliest and cheapest connections to your destination. The service can be taken on-the-go when offline via their app for iOS and Android.

There are some aggregators that claim to compare more than one mode of transportation, often including bus, train, plane and even ferry or boat. Those are usually bad at finding the best price and you are usually better off to take their results as a first ballpark estimate rather than gospel truth. That said, they can cue you as to which companies are among those plying a certain route.

Large hotels and hotel chains have a variety of rates: prepaid, flexible, corporate, discounted, etc., and they adjust their rates based on demand. Some hotels sell rooms to these booking sites at a discount. The booking companies then price in their costs to determine the price they will offer you.

Some hotel chains promise you the lowest price if you book through their website. For various reasons, many hotels consider aggregators a necessary evil at best and while you might indeed get the lowest price when booking through a certain aggregator, some hotels treat customers who booked directly through their own booking channels better, especially in terms of discretionary services not explicitly part of your contract with the hotel. Hotel owners and especially small guesthouse owners are often happy if you tell them you found them online, but came in without booking, although many hotels have higher rates for those who didn't book. Another option is to tell the hotel directly that you see a given price on a consolidator and ask them to match the price.